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See Inside November 2009

How Noise Can Help Quantum Entanglement

What spoils quantum entanglement can also restore it

Wouldn’t it be nice to be an electron? Then you, too, could take advantage of the marvels of quantum mechanics, such as being in two places at once—very handy for juggling the competing demands of modern life. Alas, physicists have long spoiled the fantasy by saying that quantum mechanics applies only to microscopic things.

Yet that is a myth. In the modern view that has gained traction in the past decade, you don’t see quantum effects in everyday life not because you are big, per se, but because those effects are camouflaged by their own sheer complexity. They are there if you know how to look, and physicists have been realizing that they show up in the macroscopic world more than they thought. “The standard arguments may be too pessimistic as to the survival of quantum effects,” says Nobel laureate physicist Anthony Leggett of the University of Illinois.

In the most distinctive such effect, called entanglement, two electrons establish a kind of telepathic link that transcends space and time. And not just electrons: you, too, retain a quantum bond with your loved ones that endures no matter how far apart you may be. If that sounds hopelessly romantic, the flip side is that particles are incurably promiscuous, hooking up with every other particle they meet. So you also retain a quantum bond with every loser who ever bumped into you on the street and every air molecule that ever brushed your skin. The bonds you want are overwhelmed by those you don’t. Entanglement thus foils entanglement, a process known as decoherence.

To preserve entanglement for use in, say, quantum computers, physicists use all the tactics of a parent trying to control a teenager’s love life, such as isolating the particle from its environment or chaperoning the particle and undoing any undesired entanglements. And they typically have about as much success. But if you can’t beat the environment, why not use it? “The environment can act more positively,” says physicist Vlatko Vedral of the National University of Singapore and the University of Oxford.

One approach has been suggested by Jianming Cai and Hans J. Briegel of the Institute for Quantum Optics and Quantum Information in Innsbruck, Austria, and Sandu Popescu of the University of Bristol in England. Suppose you have a V-shaped molecule you can open and close like a pair of tweezers. When the molecule closes, two electrons on the tips become entangled. If you just keep them there, the electrons will eventually decohere as particles from the environment bombard them, and you will have no way to reestablish entanglement.

The answer is to open up the molecule and, counterintuitively, leave the electrons even more exposed to the environment. In this position, decoherence resets the electrons back to a default, lowest-energy state. Then you can close the molecule again and reestablish entanglement afresh. If you open and close fast enough, it is as though the entanglement was never broken. The team calls this “dynamic entanglement,” as opposed to the static kind that endures as long as you can isolate the system from bombardment. The oscillation notwithstanding, the researchers say dynamic entanglement can do everything the static sort can.

A different approach uses a group of particles that act collectively as one. Because of the group’s internal dynamics, it can have multiple default, or equilibrium, states, corresponding to different but comparably energetic arrangements. A quantum computer can store data in these equilibrium states rather than in individual particles. This approach, first proposed a decade ago by Alexei Kitaev, then at the Landau Institute for Theoretical Physics in Russia, is known as passive error correction, because it does not require physicists to supervise the particles actively. If the group deviates from equilibrium, the environment does the work of pushing it back. Only when the temperature is high enough does the environment disrupt rather than stabilize the group. “The environment both adds errors as well as removes them,” says Michal Horodecki of the University of Gdansk in Poland.

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